This invention relates to diffraction-grating spectrographs. More particularly, this invention relates to concentric diffraction-grating spectrographs with modified arrangements of their optical components.
An optical spectrograph collects light at its entrance slit and forms an image of the entrance slit in the exit plane at the wavelengths present in the light source. Diffraction-grating spectrographs use one or more diffraction gratings to diffract light into specific wavelengths and to select a predetermined portion of the wavelengths present in that light.
Concentric spectrographs are particularly well suited for applications requiring sharp spectral and spatial imaging. One type of concentric spectrograph includes an entrance port, an exit port, a hemispheric field lens, and a concave diffraction grating that has a set of substantially parallel grating lines, or grooves. A concave grating has a reflective grating surface ruled on a concave surface, usually spherical, that disperses the light and focuses the spectrum. One advantage of such a grating is that it can be used without separate collimating optics. A concave grating inherently has an optical axis, which is a line that passes symmetrically through the center of curvature of the grating surface, and a meridian plane, which is a plane that contains the grating optical axis and that is substantially perpendicular to the grating lines. The entrance and exit ports of a conventional concentric spectrograph are positioned substantially in the meridian plane of the grating.
A conventional concentric spectrograph operates as follows. First, a light beam enters the spectrograph through the entrance port, which is substantially in the meridian plane of the grating. After passing through the entrance port, the light beam passes through the hemispherical lens, which causes the beam to diverge and form an expanding light cone. The longitudinal axis of that expanding light cone lies in the meridian plane of the grating. Next, the light beam is reflectively diffracted by the grating surface toward the lens in the form of a contracting light cone. The longitudinal axis of the contracting light cone also lies in the meridian plane. Once the light is incident on the surface of the lens, most of the light is transmitted by the lens and focused at the exit port for spectral analysis.
There are a number of advantages to the concentric spectrograph configuration over other known configurations, such as the Czerny-Turner configuration. First, concentric spectrographs form sharp images due to the inherent absence of Seidel aberrations. Second, concentric spectrographs can be designed with relatively large numerical apertures (e.g., numerical apertures greater than 0.7 are possible). Third, concentric spectrographs are anastigmatic, flat field devices in which linear dispersion is a function of groove density and wavelength. Fourth, concentric spectrographs provide equal magnification along and across the dispersion, which is important for convolution applications. Last, concentric spectrographs do not require the use of aspherical optical surfaces, which are relatively expensive.
Conventional concentric spectrographs, however, have a number of disadvantages. First, concentric spectrographs have difficulty preventing stray light from contaminating with the desired spectrum at the exit port of the spectrograph. Stray light may arise from a number of sources. Of particular concern is light which has been reflectively diffracted twice. Some of that light is directed toward the exit port, which appears as a faint undesirable spectrum that overlaps with the desired spectrum, a phenomenon called spectral overlap.
When a detector is placed at the exit port of a conventional concentric spectrograph, the desired and undesired spectra are both recorded by the detector. Therefore, the intensity of the desired spectrum cannot be measured independently from the undesired spectrum. In order to prevent undesirable spectra from polluting the desired spectrum, one or more filters may be placed along the optical path of the light beam, especially between the exit port of the spectrograph and the detector. Filtering, however, only works when the stray light has a different quality than the desired light, such as a different wavelength or polarization. When the wavelengths of the stray and spectral light are the same, conventional wavelength filtering techniques will not work. In any case, filtering reduces the intensity of the desired spectrum, which reduces the throughput of the spectrograph. Therefore, it would be desirable to provide a high throughput concentric spectrograph, which also has high stray light rejection without the use of filters.
Another common disadvantage of conventional concentric spectrographs is their relatively large F-numbers. As used herein, the term "F-number" refers to the ratio of an equivalent focal length of a lens to the diameter of its entrance pupil. When the F-number of a spectrograph is large, the solid angle in which light can enter the spectrograph is relatively small, which limits the throughput of the spectrograph. Also, spectrometers having large F-numbers require relatively long focal lengths, which make the instrument large.
It would therefore also be desirable to provide a concentric spectrograph that is compact, relatively inexpensive to manufacture, and relatively immune to miscalibration.
It would also be desirable to provide an inexpensive concentric spectrograph that provides an anastigmatic image at the exit port with reduced stray light.